Engineering cell-permeant FLP recombinase for tightly controlled inducible and reversible overexpression in embryonic stem cells.

Combined application of DNA recombinases Cre and FLP enables tightly controlled independent and/or sequential gene regulations. However, in practice, such dual recombinase strategies are hampered by the comparably low efficiency of the FLP recombinase. Here, we present the engineering of a recombinant cell-permeant FLP protein (TAT-FLP) that induces recombination in >75% of fibroblasts and mouse as well as human embryonic stem (ES) cells. We show that TAT-FLP ideally complements the strength of cell-permeant Cre recombinase for genetic engineering as exemplified by FLP-ON-Cre-OFF, an inducible transgene expression cassette that enables tightly controlled expression in a reversible manner. We exemplify this concept by conditional overexpression of LacZ and the caudal-related homeobox transcription factor CDX2. We expect our FLP transduction system to become widely useful for numerous genetic interventions addressing complex biological questions and the generation of transgene-free therapeutically applicable ES cell-derived cells.